1. Field of the Invention
The present invention is generally directed toward rupture discs, specifically reverse acting discs, having a machined line of opening formed therein. The line of opening presents as a recess and is formed by a mechanical milling operation whereby metal is removed from the bulged portion of the disc without altering or disrupting the substantially uniform metallic grain structure of the metal adjacent the recess.
2. Description of the Prior Art
It has long been known to provide bulged reverse acting rupture discs having a line-of-weakness recess or score line in one face of the disc bulge. The lines of weakness or score lines have generally been cross scores, or a circumferential line-of-weakness recess in the concave face of the disc where the line-of-weakness recess or score line defines the area of the disc that opens upon reversal. Without a line-of-weakness recess defining the opening through the disc upon severing of the disc along the line-of-weakness recess, a bulged disc will reverse but not necessarily fully open. In the case of a circumferentially extending line-of-weakness recess, the line of weakness normally is not a continuous line, thus presenting a hinge area that prevents fragmentation of the central area of the disc upon reversal and opening. A cross scored disc forms four petals that bend outwardly upon reversal of the disc, again preventing fragmentation of the petals. Circumferential score lines or lines of weakness are preferred in low pressure applications because of the larger opening presented upon severing of the disc along the arcuate score line, as compared with a cross scored disc.
Lines of weakness have heretofore been formed in reverse acting rupture discs by a metal scoring die, use of a laser that erodes a groove in the disc, or by chemical etching or electropolishing to remove metal from the disc along a desired line. All of these past reverse acting discs have presented unresolved manufacturing difficulties, or have experienced operational problems in various application uses.
Metal scoring dies work harden the metal material, thus changing the grain structure and density of the metal at the score line. The material surrounding a score line formed with a metal scoring die is work hardened during the scoring process, thus increasing the brittleness of the metal and creating stress zones. The brittleness and increased stress zones of the metal limit the service life of the rupture disc as a result of fatigue cracking and stress corrosion. Metal scoring depths required for satisfactory operation profoundly alter the original bulged dome strength making it difficult to predict the pressure ultimately required to reverse the rupture disc during the initial bulging operation of the disc prior to scoring. Consequently, it is very difficult to produce a reverse acting bulged rupture disc having a score line formed with a scoring die that will both open reliably and withstand multiple successive pressure cycles.
It has also been proposed to form a score line in a reverse acting rupture disc using a laser beam. These proposals have not proved commercially satisfactory for a number of reasons. The reflectivity of the metal makes it difficult to control the penetration of the beam into the thickness of the metal and thereby form a smooth groove of uniform depth along the length of the intended line-of-weakness recess. Furthermore, lasers significantly heat and burn the disc, oxidize the material and change the metallurgy of the metal. Discs having lines of weakness burned by a laser have been found to be unsatisfactory in use, not only from the standpoint of unreliable openings at required pressure relief values, but also having undesirable cycle life.
Chemical etching of a rupture disc having a segmented resist layer defining a line of weakness has also been suggested in the prior art, as for example shown and described in U.S. Pat. Nos. 4,122,595, 4,597,505, 4,669,626, and 4,803,136. The patentee in the '595 patent suggests screen printing of a resist material on a flat rupture disc where the screen has openings presenting a pattern of the desired line of weakness. After bulging of the disc, an acid solution is sprayed onto the disc to etch a line of weakness coincident with the area of the disc unprotected by the resist material. The metal surface of the disc material is somewhat irregular and not perfectly smooth because the individual side-by-side grains have peaks with valley structure between the grains. Therefore, when an acid etchant agent is applied to the surface of the metal, that agent does not act uniformly across the surface of the metal. Instead, the etchant is more aggressive in the valleys between the grains than in eroding the higher surface peaks of the metal grains. The etchant agent contained in the valley cavities is believed to not only more rapidly erode the metal in the valley area as compared with the surrounding peak areas of the grains, but to also do so more efficiently. The attendant result of the etching process is to exaggerate the roughness of the metal surface, with the degree of surface irregularity increasing with time of exposure of the metal to the enchant agent. The rupture discs are manufactured from materials that are inherently corrosion resistant, such as stainless steel, INCONEL, Hastalloy-C, and MONEL. As a consequence, subjection of these inherently corrosion resistant materials to an etchant acid requires that the etchant agent remain in contact with the surface of the metal for extended periods of time in order to erode away a groove that typically is as much as 70-90% of the thickness of the metal. For example, if the material is 0.004 in. thick, as much as 0.0036 in. must be eroded during the etching process.
In addition, in order to accomplish reasonably efficient erosion of these corrosion resistant materials, the etchant agent chosen must be one tailored for the particular type of metal. Thus, a different acidic agent is required for each of the various metals. The specific material used to manufacture a particular rupture disc must be selected to meet the specifications of the application. Different disc applications require use of different types of metal. Therefore, when an etching process is used to form a line of weakness in corrosion resistant disc material, the manufacturer should have available an etchant agent that is most effective in eroding that specific corrosion resistant metal.
U.S. Patent Application Publication 2006/0237457 discloses forming a laser defined, electropolished line of weakness in a reverse acting rupture disc. A rupture disc blank is first pre-bulged, final bulged, and then provided with a layer of resist material. A laser is used to remove at least a portion of the layer of resist material corresponding to a desired line-of-weakness recess in the concave face of the bulged rupture disc. The disc is then subject to an electropolishing operation to remove metal from the lased area of the rupture disc, thereby forming a lustrous polished line-of-weakness recess in the disc. This technique produces acceptable results when used in connection with relatively thin, small-diameter discs. However, as the thickness and diameter of the disc increases, the electropolishing technique can become a less attractive means of forming the line-of-weakness recesses. Discs formed of thicker materials generally require longer electropolishing times in order to achieve a recess having the desired depth. The longer electropolishing times also result in an increase in the width of the recess. If the recess becomes too wide, the recess may begin to affect the burst pressure of the disc thereby resulting in a loss of control over the disc's opening characteristics.